1. Field of the Invention
The present invention relates to a NOx sensor and a method of measuring NOx. In particular, the present invention relates to improvement in a sensor directed to a combustion gas as a measurement gas, for measuring NOx as a measurement gas component in such a gas, and a method capable of advantageously measuring NOx.
2. Description of the Related Art
Various measuring methods and devices have been proposed for determining the concentration of NOx in a measurement gas. A known method, for example, employs a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia. This method utilizes the ability of Rh to reduce NOx so that an electromotive force generated between the two electrodes is measured. However, a problem arises in that such a sensor tends to suffer from an influence of noise, since the electromotive force varies to a great extent depending on a change in concentration of oxygen contained in a combustion gas as a measurement gas, while the electromotive force varies to a small extent in response to a change in concentration of NOx. On the other hand, a reducing gas such as CO is indispensable for such a sensor in order to induce the ability to reduce NOx. However, in general, a large amount of NOx is produced under a combustion condition concerning an excessively small amount of fuel, in which an amount of produced CO is less than the amount of produced NOx, resulting in a drawback that measurement cannot be performed for a combustion gas discharged under such a combustion condition.
Another method of measuring NOx is also known, based on a combination of a set of electrochemical pumping cell and sensing cell including Pt electrodes and oxygen-ion conductive solid electrolyte, and another set of electrochemical pumping cell and sensing cell including Rh electrodes and oxygen-ion conductive solid electrolyte, as disclosed in Japanese Laid-open Patent Publication Nos. 63-38154 and 64-39545. In this method, NOx is measured on the basis of a difference between values of pumping currents. Other methods are disclosed, for example, in Japanese Laid-open Patent Publication Nos. 1-277751 and 2-1543. In one method, two pairs, i.e., a first pair and a second pair of electrochemical pumping cell and sensing cell are prepared. A limiting pumping current is measured by using a sensor comprising the first pair of pumping and sensing cells, under a partial pressure of oxygen at which NOx is not reduced, while a limiting pumping current is measured by using a sensor comprising the second pair of pumping and sensing cells, under a partial pressure of oxygen at which NOx is reduced, so that a difference between the measured limiting pumping currents is determined. In another method, a sensor comprising a pair of pumping cell and sensing cell is used, in which a difference in limiting current is measured by switching the partial pressure of oxygen in a measurement gas between a partial pressure of oxygen at which NOx is reduced and a partial pressure of oxygen at which NOx is not reduced.
In the aforementioned methods of measuring NOx, however, an extremely small part of the value of the limiting current is based on the objective NOx, and the most part of the value of the limiting current is occupied by electric power caused by oxygen contained in a large amount in ordinary cases. Therefore, a small current value corresponding to NOx is determined from a difference between two large current values. Accordingly, in the case of the method based on the use of the one set of sensor, problems arise in that the NOx cannot be continuously measured, the operating response is inferior, and the accuracy is inferior. On the other hand, in the case of the method based on the use of the two sets of sensors, an error is likely to occur in a measured value if the oxygen concentration in a measurement gas greatly changes. Therefore, this method cannot be employed in automobile applications, for example, where the oxygen concentration in a measurement gas varies to a large extent. This inconvenience arises from the fact that the dependency of pumping current on oxygen concentration concerning one sensor is different from that concerning the other sensor. In the case of an automobile, for example, the oxygen concentration in exhaust gas is generally several percentages under a running condition of an air/fuel ratio of 20, whereas the NOx concentration is several hundreds of ppm. The concentration of NOx is about 1/100 of the concentration of oxygen. Therefore, only a slight difference in the dependency of pumping current on oxygen concentration brings about a situation in which a difference in the limiting current value corresponding to a change in oxygen concentration is larger than an amount of change in the limiting current based on NOx to be measured. In addition, if a diffusion rate-determining means formed in the pumping cell is clogged with oil ash in the exhaust gas, the pumping current may be undesirably changed, resulting in reduced accuracy. Further, if the temperature of the exhaust gas greatly varies, a measured value may involve some abnormality. Moreover, a difference in chronological change in any characteristic between the two sensors, if any, may directly lead to measuring errors, resulting in a drawback that the entire system is made undurable for use over a long period of time.
The oxygen present in the measurement gas causes various problems upon measurement of NOx, as described above. Accordingly, it has been strongly desired to solve these problems.
In order to solve the problems described above, the present inventors have revealed a new measuring system in Japanese Patent Application No. 7-48551. In this system, a measurement gas component having bonded oxygen, such as NOx, contained in a measurement gas can be measured continuously and accurately with good response over a long period of time without being affected by the oxygen concentration in the measurement gas or any change thereof, by utilizing first and second electrochemical pumping cells arranged in series.
Namely, in the previously proposed new system, a measurement gas containing a measurement gas component having bonded oxygen to be measured is successively introduced from an external measurement gas-existing space into first and second processing zones under predetermined diffusion resistances respectively. At first, in the first processing zone, the partial pressure of oxygen is controlled to have a low value which does not substantially affect measurement of an amount of the measurement gas component by pumping out oxygen in the atmosphere by using a first electrochemical pumping cell. In the second processing zone, the measurement gas component in the atmosphere introduced from the first processing zone is reduced or decomposed. Oxygen produced by the reduction or decomposition is pumped out by the aid of an oxygen-pumping action effected by a second electrochemical pumping cell. Thus a pumping current flowing through the second electrochemical pumping cell is detected to obtain a detected value from which the amount of the measurement gas component in the measurement gas is determined.
However, as a result of further investigation by the present inventors on such a new measuring system, the sensor concerning the new measuring system described above has been clarified to have the following problem. Namely, measurement of a measurement gas component at a low concentration, for example, measurement of NOx at several ppm gives a pumping current of about several tens of nA detected by the second electrochemical pumping cell, which is small as a detection signal.